Isolation of intracytoplasmic A particles from primary and transplanted mouse mammary tumors. Biochemical, immunologic, ultrastructural, and biologic characterization of the purified particles and comparison of those results with those obtained with similarly purified mouse mammary tumor virus (MMTV). A particle proteins were isolated from SDS-polyacrylamide gels, three MMTV antigenic determinants P27, P14, P10 were demonstrated to be associated with A particle structural polypeptides by immunodiffusion. A precursor-product relationship between A particles and MMTV nucleoprotein cores is becoming apparent. Single stranded DNA found bound to A particles in situ is not apparently complementary to viral RNA or to MMTV cDNA (CAP). Studies are in progress to determine the relationship of this DNA to mouse unique and related sequences and to normal cell cytoplasmic DNA(s). Chemical viral co-carcinogenesis in non-MMTV-expressing C3H/StWi mice is also under study.